Barrel Suck- BS?

[HunterAndy]

Barrel Suck, BS or Fact?


I’ve been looking at the whole Barrel Suck phenomenon and I just can not accept it as given.
I needed proof. So I decided to formulate some calculations as a starting point.

I’ll cover a lot more in this post, so please read through it fully before posting knee jerk reactions to my hypothesis.

For the purpose of this, I need to clarify some terms. Most will know them, but just in case…

Definitions;

AEG: Automatic Electric Gun (airsoft)

Barrel Suck: The phenomenon that apparently occurs when an AEG piston begins retracting while a BB is still in the barrel, causing a loss in FPS due to the piston sucking back air from the barrel behind the BB.

FPS: Feet per Second – Velocity of the airsoft projectile

IPS: Inches per Second (FPS X 12) used for calculations within this hypothesis.

ROF: Cyclic rate of fire measured in Rounds per Second or RPS

My hypothesis:
Barrel Suck is a misnomer perpetrated on the Airsoft Community at large due in large part to certain active persons in forums across the world coining the buzz phrase with little more than speculation and guesswork and a great degree of “Technical Gobbledegook” to fool the masses into a perpetual belief that what they say must be true.


Here are some mathematical statistics and formulas that can be duplicated to match any guns specific performance values. This is assuming the AEG is in prime operating condition and NOT double feeding due to magazine or hop-up issues.


Test Bed #1 (Long barrel, Low FPS, High ROF):

Inner Barrel Length : 600mm (600mm X 0.03937 = 23.622 in, but let’s call it 24”)

FPS: 250 (about the lowest FPS for a decent gun)

Cyclic Rate: 20 RPS

Calculations:
At 250 FPS, it takes a BB 0.008 seconds to travel 2 feet (24”, or just over 600mm).
[1 second, divided by 250 feet, times 2 feet]

At 20 RPS, a BB is fired once every .05 seconds.
[1 second divided by 20]

Assuming Zero Point on our timeline to be the point where the piston releases from the sector gear, the BB exits the barrel at approximately 0.008 seconds, leaving 0.042 seconds before the next shot even begins to travel down the barrel.
[0.05 minus 0.008]

The BB is only in the barrel for the first 16% of the firing cycle.


Test Bed #2 (Long barrel, High FPS, Moderate ROF):

Inner Barrel Length : 600mm (600mm X 0.03937 = 23.622 in, but let’s call it 24”)

FPS: 400 (the golden number for many)

Cyclic Rate: 15 RPS

Calculations:
At 400 FPS, it takes a BB 0.005 seconds to travel 2 feet (24”, or just over 600mm).
[1 second, divided by 400 feet, times 2 feet]

At 15 RPS, a BB is fired once every .066 seconds.
[1 second divided by 15]

At Zero Point (piston release), the BB exits the barrel at approximately 0.005 seconds, leaving 0.061 seconds before the next shot even begins to travel down the barrel.
[0.066 minus 0.005]

The BB is only in the barrel for the first 7.5% of the firing cycle.


Test Bed #3 (Average barrel, Average FPS, Moderate ROF):

Inner Barrel Length : 363mm (363mm X 0.03937 = 14.29 in, but let’s call it 14.5”)

FPS: 350 (pretty standard for non TM stock guns)

Cyclic Rate: 15 RPS

Calculations:
At 350 FPS, it takes a BB 0.0035 seconds to travel 14.5 inches.
[1 second, divided by 4200 IPS (350 FPS X 12), times 14.5]

At 15 RPS, a BB is fired once every .066 seconds.
[1 second divided by 15]

At Zero Point (piston release), the BB exits the barrel at approximately 0.0035 seconds, leaving 0.0625 seconds before the next shot even begins to travel down the barrel.
[0.066 minus 0.0035]

The BB is only in the barrel for the first 5.3% of the firing cycle.

Based on these examples, anywhere from 84 to 94.7% of the time (while the gears are spinning) there is nothing going down the barrel!



Also busting this “Barrel Suck” myth is the actual function of the AEG gearbox..

Before the sector gear teeth can even engage the piston teeth, the tappet plate is disengaging the nozzle from the hop-up chamber thanks to the ingenious location of the tappet nub.
While the piston is retracting, it will draw air through the nozzle, but since the nozzle is no longer sealed within the hop-up chamber, it can not suck air from the barrel. Try sucking soda through a straw with the straw opening just millimeters outside of the liquid. All you’ll get is air. Your sucking has no affect on the soda, just like the piston has no affect on the BB in the barrel (if it’s even still IN the barrel).



Now, with all of that being said, there is a problem which occurs with improper cylinder sizing which will affect FPS, Accuracy, Consistency, and Performance.

This problem occurs when your cylinder does not physically have enough air volume to fully propel a projectile down the entire length of the barrel.

Typically, cylinders have venting holes at certain positions along the length of the body. This is primarily to minimize air resistance during piston acceleration after release in small to medium barrel lengths. There are many resources available to guide you to the proper cylinder type for your barrel length.

Putting a 500mm long barrel in a gun with a cylinder designed for a 363mm barrel is looking for trouble. Your cylinder will not have the sufficient air volume to propel the projectile down the entire length of the barrel.

Use the following formula:

Cylinder ID in mm X Cylinder ID in mm X 2.1415 (Pi) X Cylinder length in mm (to hole, if it has one) = Cylinder Volume in Cu mm

Barrel ID in mm (if you don’t know the ID, use 6.08) X Barrel ID in mm X 2.1415 (Pi) X Barrel Length in mm = Barrel Volume in Cu mm
If your Cylinder Volume exceeds your Barrel Volume, you’re good to go.
If it’s close, you may want to move to a Zero or Bore Up cylinder.


So, in closing, the phenomenon known as “Barrel Suck” has absolutely nothing to do with barrel suck. It is simply a result of improper sizing of cylinder type to barrel length.

Credit to the guy who has actually spent a bit of time "disproving" the hallowing myth of back suck.

Now I'm not saying this guy is speaking gospel.
But having raed through it, his logic seems VERY reasonable.

This theory seems to state you can have a mismatched barrel and cylinder, but you can't ever get backsuck.

Anyone else like to comment on this, I previously thought it was a possibility but I didn't ever have any problems for it.

The only time I did have a problem was when my tappet plate was broken and the airseal caused about a 20 fps shot. :D


However he did not make the calculations that the BB's must accelerate up to 250fps from a standstill in the distance.

[shadowcaptain]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

never heard of this - interesting read but i dont think it will ever really cause a problem

(also you have nothing better to do at 00:30- actually then again i am sat here reading it)

[Taylorb]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

Ignore this is something different (feels stupid)

[Taylorb]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

What is confusing though, is that he gives this formula for calculating the volume of the cylinder:

"Cylinder ID in mm X Cylinder ID in mm X 2.1415 (Pi) X Cylinder length in mm (to hole, if it has one) = Cylinder Volume in Cu mm"

But the volume of a cylinder is: (Pi)radius(squared) times the length of the cylinder. So that would mean:

Cylinder ID/2 (squared) x Pi (3.141 not "2.1415") x the length of cylinder. Confusing or am i missing something here?

*He does the same thing for barrel length too, by using: d(squared)x(pi)xlength instead of: (Pi) x radius(squared) x length.

Is (Pi) different where he comes from? I am confused :p

[HunterAndy]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

I've just done a bit of thinking myself, and he has done the assumption that a BB's initial velocity on the first example would be 250fps.

This makes all his following assumption wrong:
1) The time taken is not a simple function of distance anymore, as the BB is constantly accelerating to the point it leaves the barrel.

However This can be partly rectified by the mechanics dynamic equation s = 1/2(u+v)t

where s = barrel length, u is initial velocity = 0, v is muzzle velocity = 250fps and t = time.

For ease sake I converted one into metric functions as I work better with them myself.

It worked out that using the first example, a BB will exit the barrel in 0.016 seconds after the piston is released. (I cba to write up the math, use the equations yourself if you don't trust my working )

Now a Full cycle is 0.05 seconds, leaving 0.036 seconds for a drawback of a piston. Now this is more like 28% of the total firing cycle, rather than his original 16%.

What begs the question is what would the drawback time of that set up be, in theory it wouldn't be the full 72% because the Piston must first fully empty the cylinder of air, then be picked up by the gear and drawn back again, 2 unknown times for his setup.


There was another post joking about canadians not knowing pi, but it was just a typo I do believe.

[Taylorb]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

You seem to be right mate, but it's getting far too late for me to fully comprehend all of this. It certainly seems to be full of holes. Once i read the formula at the bottom and realised it was all wrong I gave up on it :p

I am reading this right though?

He gives the Cylinder volume as: d(squared)x(pi)xlength whereas it should be r(squared)x(pi)xlength ... as in pi-r-squared.

right?

[HunterAndy]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

Yeah you are right he doesn't know as much as I first thought

Perfect example of why not to skim read

, the main reason why I was posting this is I was wondering if suck back is indeed possible in a regular set up.


This guy has had an attempt at debugging the myth, but not done so.

I was wondering if anyone more experienced has got any more details about its possibility, and perhaps some factual evidence to back it up.

[chrisn92]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

Taylor you are correct, thats the same equation that we used in physics.

[Taylorb]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

I don't really know about it, I was under the impression that if your cylinder volume was greater than your barrel volume in a vsr then you could get 'suck back.' However I had never really thought about it in an AEG.

Also HunterAndy your math seems to be correct, and he hasn't factored acceleration of the bb along the barrel, so his percentages are flawed.

There's a few techies on here that may have a better idea though...

[omgitzjamiiee]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

can someone please put all that technical crap into English please? dose it suck or not?

[shadowcaptain]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

yes...im with jamie on this

while im quite good at maths im far too tired to even read this post now

[chrisn92]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

Jamie and shadow, his theory has been proved wrong and therefore it is unknown.

[shadowcaptain]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

wow so we have gotten nowhere....

sorry ill stop posting now haha i should sleep

[Taylorb]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

can someone please put all that technical crap into English please? dose it suck or not?

Well after looking at the guys tests it turns out he wasn't accounting for all the necessary factors and that his formulas were wrong, so his conclusions were unreliable.

So the answer is still unclear...

[Taylorb]

Re: Interesting post on evil other forum i found- Barrel Suck- BS?

wow so we have gotten nowhere....

And yes we have managed to get no where number-crunching at 1 o'clock in the morning

*Just to point out, but if this guy put so much time into this test and writing up the thread you would have thought he would, at the very least, have checked over his formulas and typos. I would have certainly double checked before posting a controversial thread like that.